There are enormous difficulties in reconstructing anyone's life, however copious the evidence of letters and journals. Moods and emotions are volatile, and when recorded on the page are often forced by posterity to carry a much greater weight than was ever intended by their author. In her book on EM Forster, Rose Macaulay criticised his unsympathetic response to a letter of Jane Austen's. Austen wrote to her sister Cassandra: "Mrs Hall was brought to bed yesterday of a dead child, some weeks before she expected, owing to a fright. I suppose she happened unawares to look at her husband." Forster disliked the unkindness of this; he imagined Jane and Cassandra laughing together over unfortunate Mrs Hall. But Cassandra's reaction might have been very different, Macaulay pointed out. She might have chided Jane for unkindness and Jane might have repented. "That is the worst of publishing the letters of the dead," concluded Rose. "They grin and stare and grimace and scowl at us, expressing for ever, in black ink on paper, moods which were scarcely even moods, so glibly did they run by, run off the pen."

Rose Macaulay published her first novel, Abbots Verney, in 1906, and went on to produce 22 more. In the 1920s she began to make formal experiments with structure and voice, and to play with thematic concerns that came to be associated with the modernist movement; these novels, which include Told by an Idiot and Keeping Up Appearances , were hugely successful. After her death in 1958, three volumes of her letters were published, followed by a biography in 1972 and another one in 1991. The second biography, by Jane Emery, presented a solidly literary Rose Macaulay, but it is byher correspondence that she is better known, although her published letters were addressed to only two recipients. These were Father Hamilton Johnson, a member of the Cowley Fathers in the US, with whom Macaulay began corresponding in 1950, and her sister Jean, a nurse and a devout Anglican. Johnson was instrumental in helping Rose re-enter the Anglican church after many years of self-imposed exile. She had left in the early 20s, choosing instead to continue her love affair with the married Irish novelist Gerald O'Donovan. Jean too disapproved of Rose's affair; if it was ever referred to in their correspondence (which it may well not have been), those references were deleted. On her own admission, Jean had done a lot of burning before handing over the letters to Constance Babington Smith, a distant cousin (and Rose's first biographer), to edit for publication.

So while Macaulay's concern over the finer points of liturgical history, her passionate prejudices against Roman Catholics, and her obsession with the rel ative speeds of air and sea mail are exhausively documented in her published letters, there is very little about Gerald O'Donovan. There are no photographs of them together, and very few surviving letters even mention his name. None of hers to him survives; nor his to her, for in May 1941 she lost everything when her central London flat was bombed. It's not easy to piece together a sense of their relationship, and the significance of it to her life as a writer.

Rose was notorious for refusing to divulge the tiniest personal detail to journalists or critics; she called one of her novels I Would Be Private . O'Donovan too was intensely secretive. His first novel, Father Ralph (1913), is a semi-autobiographical account of the life of an idealistic young priest in turn-of- the-century Galway. But his own past as Father Jeremiah O'Donovan, parish priest in the west of Ireland, was not known to his children until they were well into adulthood. Rose met him in 1918, when she was transferred from the War Office to the Italian section of the department of enemy propaganda in the newly set-up Ministry of Information, which he ran. He had been married for nearly eight years, and his wife was about to give birth to their third child.

There are conflicting accounts of the beginning of the relationship. Rose's sister Jean maintained that Gerald did not tell Rose he was married until she was in love with him. This is what happens to Rome Garden in Told by an Idiot (1923); but it is possible that Jean came to her conclusion by inferring from the fiction that that was how it happened. Gerald took his family to live in Italy for most of 1920, which suggests that he was struggling, but in 1921 a choice was made. The price that Rose paid was her spiritual life within the Anglican communion, and her intimacy with her sisters Margaret and Jean, who viewed her choice as "a sad tragedy". But for her life as a writer it was a price worth paying. She could have love without loss of independence and without the burden of domestic responsibility.

"I always talked over my novels with my companion, who stimulated my invention," said Rose. Gerald worked for William Collins - Rose's publisher from 1920 - as one of their chief readers. At first they were seen around together in literary London but Rose soon awoke to the hurtful power of malicious gossip - a theme of Crewe Train (1926) - and from then on they vanished as a couple from public view. Years later Victor Gollancz referred to their love affair as "one of the best-kept secrets in London".

A pattern was established that allowed Rose into Gerald's home life: throughout the 20s and 30s she would have lunch every other Sunday at his house in Surrey, and she became an honorary aunt to his three children. Trips abroad together also became more frequent. Gerald needed to make no excuses to his family for his frequent disappearances. He brought up his children in the belief that he was a target for the ire of the powerful Catholic establishment, and that they must never know what work he was engaged on for fear they would "let it out", which seems a remarkably convenient position for a man who wants to be away from home with no questions asked.

Gerald stopped writing novels in 1922. Of his sixth and final novel, The Holy Tree, Rose wrote to Rosamond Lehmann: "In it he puts his whole philosophy of love ... all the things he would say to me about love and life, all he felt about me, all we both knew." After he left Ireland, Gerald was forever haunted by a feeling of unbelonging. Just as his desire for secrecy spoke to Rose's, so his outsiderness offered a reflection for hers: outside the mainstream, like Laurie in Rose's last novel, The Towers of Trebizond (1956), gazing at the towers shimmering in the distance.

Because of the secrecy surrounding their relationship, it's perhaps not surprising that Gerald seems most present in Macaulay's work when he is absent and she is mourning his loss. He is painfully present in "Miss Anstruther's Letters", a short story written soon after her flat was destroyed. In the story Miss Anstruther's lover is recently dead, and her flat has been bombed. So raw is Miss Anstruther's grief that she has been unable to reread her lover's letters, but has been saving them for a time when she would be able to without giving way to despair. Now she will never read them again. Only one scrap of paper has escaped the flames: written on it one hurtful phrase that turns all the years of passion into "a drift of grey ashes". As a result Miss Anstruther, too, becomes "a drifting ghost". When Rose Macaulay wrote the story, Gerald O'Donovan was not dead but dying. She was able to ask him to replace her precious copy of The Holy Tree , but the letters, the written record of the love that flourished in their "secret stolen travels of twenty years", could not be replaced.

After his death Rose found it impossible to write fiction for almost a decade, but instead wrote a book about Spanish history, Fabled Shore. She had explored the past before, notably in her 17th-century historical novel They Were Defeated and in her biography of John Milton, but her journeys into the past after Gerald's death are different. In Fabled Shore, in The World My Wilderness (1950), her first novel for 10 years, which is set in the blitzed ruins of the City of London, and in her huge work Pleasure of Ruins , she is fascinated by the way the voices of the dead speak through the ruins of the streets and houses and buildings they once inhabited. O'Donovan haunts the pages of the books Rose published in the 40s and 50s, present in a way that he is not present in her published letters. "Asking questions," said VS Pritchett of Rose after her death, was "her ironical pleasure"; and it is only through asking questions of the evidence that is left that the biographer begins to uncover the range of what Daisy in Keeping Up Appearances calls a person's "queer hidden selves".

Sarah LeFanu's biography, Rose Macaulay , is published on June 5 by Virago